The disclosure relates generally to turbomachines such as steam and gas turbines, and more particularly, to a system and method for adjusting cold clearances between rotating turbine blade tips and their surrounding casing in situ.
Turbomachines, such as gas and steam turbines, typically include a centrally-disposed rotor that rotates within a stator. A working fluid flows through one or more rows of circumferentially arranged rotating blades that extend radially outwardly from the rotor shaft. The fluid imparts energy to the shaft, which is used to drive a load such as an electric generator or compressor.
Tight seal clearances between radially outer tips of the rotating blades and stationary shrouds on an interior of the stator are necessary for maximally efficient operation of the turbomachine. The smaller the clearance between the rotor blades and the inner surface of the stator, the lower the likelihood of fluid leakage across blade tips. Such fluid leakages may cause fluid to bypass a stage of blades, thereby reducing efficiency.
Differing operating conditions may cause blades and other components to experience thermal expansion, which may result in variations in blade tip clearance. The specific effects of various operating conditions on blade clearance may vary depending on the type and design of a particular turbomachine. For example, tip clearances in gas turbine compressors may reach their nadir values when the turbine is shut down and cooled, whereas tip clearances in low pressure steam turbines may reach their nadir values during steady state full load operation. Proper seal clearance during turbine operation depends on proper adjustment of cold seal clearance during turbomachine assembly, and control of the relative rotor and stator positions.
One method of adjusting relative rotor and stator positions, and therefore the relevant clearances, is the performance of iterative tops-off and tops-on clearance measurements and adjustments. During turbine assembly or re-assembly, the lower stator shell may be assembled first, followed by placing the rotor. Although rotor-to-stator clearances can be measured in the lower half prior to assembling the upper half (i.e., in the “tops-off” condition), these values may not be directly representative of the values in the fully assembled turbine (i.e., in “tops-on” condition) because the turbine shell is supported differently when the upper shell of the stator is affixed to the lower shell. In the tops-on condition, support may be shifted from the lower shell arm to the upper shell arms, the weight of the upper shell of the stator is added, and when the horizontal joint is bolted, the overall stator structure stiffens. As a result of these and other changes, the rotor-to-stator clearance may be different in the tops-on and tops-off conditions, by a factor which may not be readily predictable. Often, multiple iterations are performed, in which the upper stator shell is disassembled and reassembled in an adjusted position based on the differences in tops-on and tops-off clearances.